With the use of solar energy and other renewable energy flourishing around the world, concentrated solar power (CSP) has gradually been recognized. In a CSP system, heat absorption and heat transfer have a very important role. In solar heat collection technologies, currently heat transfer oil is mainly used as a heat transfer working medium, after heat exchange through the heat transfer oil, a conventional steam turbine is driven to make a generator set generate power. Since the operating temperature of the heat transfer oil has to be controlled below 400° C., beyond this temperature it will cause heat transfer oil cracking, increase in viscosity and heat transfer efficiency reduction and other issues, thus limiting the operating temperature of the concentrated solar power. Meanwhile, the usage cost of heat transfer oil is high, therefore, there is an urgent need for new heat transfer working medium to replace heat transfer oil, so as to raise the operating temperature and reduce the equipment cost and operating cost. Currently, alternatives of heat transfer medium of international solar heat collection technologies include molten salt materials, but their crystallization point is relatively high, most are about from 230° C. to 260° C., so there are still many difficulties in direct replacement. Currently molten salt is mainly used for heat energy storage.
Direct steam generation (DSG) technology directly using water as a heat exchange medium has been tested for many years, and the principle of the technology is similar to the operation principle of a steam boiler heat pipe, which uses water as working medium to inject low temperature water from an end of heat absorption pipeline. The temperature of the water gradually warms up during the process of moving along the axial direction of the pipeline, and the water becomes saturated vapor when reaching the boiling point, and then continues to absorb heat and becomes superheated vapor. Because instability occurs when the water boils in the heating tube, there are problems of two-phase stream transmission, uneven of vapor pressure in the heat collection tube, etc., and damage phenomena of water hammer, vibration, piping material fatigue occur; also during the period when saturated vapor becomes superheated vapor, due to the poor heat conductivity of vapor, heat absorption capacity is relatively weak and damage caused by pipe over-heat is prone to occur; and when the pipeline is unevenly heated, the temperature difference of the pipe walls is relatively large, therefore serious bending will occur, which brings other losses (such as a vacuum seal damage); in addition, the existing technology still cannot solve a series of problems caused by that a portion of the DSG pipe is not heated (for example, the mirror field has shadow locally due to cloud coverage), for example, the flow control of water input and vapor output, and impacts of parameter change. Therefore, the technology is still at the experimental stage. But as long as these problems can be solved, the DSG technology will become the key technology of environmentally safe solar thermal power generation with the lowest cost and highest efficiency.